Modern commercial electronic products often require multilayer printed circuit boards to achieve a high degree of functionality in a small volume. In recent years, the economic demands upon packaging technology to make things better, cheaper, and faster has led to the development of multilayer ceramic packages for RF integrated circuit (RFIC) semiconductor chips. These advanced devices include power distribution runs, built in resistors and capacitors, and perhaps even simple microwave components such as couplers and power combiners. Modern military phased array antenna systems require multilayer microwave corporate feed assemblies to achieve a high degree of functionality, especially at high frequencies where the array element spacing is small. These boards and packages are assembled layer by layer, and precise alignment between the separately formed layers is necessary to ensure the appropriate electrical connections are made between the various layers. When such multilayered devices are manufactured, though, it is difficult to determine that the multiple layers have been properly aligned for optimal function.
Because such misalignment occurs between buried conductor layers which are not visible to the eye, a number of partial solutions have been developed. X-rays can be used to probe the interior of the device, but such methods offer limited resolution, a waiting period while films develop, and potential health hazards associated with the use of X-rays. Another solution has been to cut apart the finished assembly to view the cross section. Obviously, this test destroys the device in question, which can be costly, and similarly presents a delay time. Application of this test to ceramic-based devices may prove even more difficult due to the nature of the materials involved. Furthermore, such a destructive test cannot be used to evaluate the quality of device prior to use or sale.
A quick, inexpensive, and accurate method for detecting layer misalignment in multilayer devices is needed. A quick, inexpensive, accurate, and safe method for detecting layer misalignment in a multilayer device that does not render the device inoperable is also needed.
The invention pertains to a coupler array that is manufactured as a part of a multilayer chip which can be used to quickly and easily detect misalignment of layers of the multilayer board. In one aspect, the invention provides a pair of coupling strips for detecting layer misalignment in a multilayer device, comprising a lower coupling strip formed on a first layer of said multilayer device, and an upper coupling strip formed on a second layer of said multilayer device disposed parallel to and offset from the lower coupling strip by a predetermined amount, such that measuring the amount of coupling in the pair of coupling strips is useful for detection of layer misalignment in the device.
In one embodiment, the invention provides a coupler array for detecting layer misalignment in a multilayer device, comprising at least two pairs of coupling strips as described above disposed in parallel, wherein the offset amounts of at least two pairs of coupling strips are different, such that measuring the amount of coupling in different pairs of coupling strips is useful for detection of layer misalignment in the device. In one embodiment, at least one pair of coupling strips has an offset amount of zero. In another embodiment, the offset amounts of at least two pairs of coupling strips differ by a predetermined amount. In another embodiment, the array may include at least two additional pairs of coupling strips disposed in parallel, such that the offset amounts of at least two additional pairs of coupling strips are different, and the additional pairs of coupling strips are not parallel to the first pairs of coupling strips. In certain related embodiments, measuring the amount of coupling in the different pairs of coupling strips is useful for determining layer misalignment in two dimensions.
In yet another embodiment, an array of coupling strips includes at least two additional pairs of coupling strips disposed in parallel, wherein the offset amounts of at least two second pairs of coupling strips differ, and wherein the upper coupling strips of said second pairs of coupling strips are disposed on a third layer of said multilayer device, and the lower coupling strips of said second pairs of coupling strips are disposed on the second layer of the multilayer device.
Another aspect of the present invention provides method for manufacturing a pair of coupling strips for detecting layer misalignment in a multilayer device, by providing a first layer of the multilayer device, forming a lower coupling strip on the first layer of the multilayer device, disposing a second layer of the multilayer device above said lower coupling strip, and forming an upper coupling strip on the second layer of said multilayer device, the upper coupling strip being disposed parallel to the lower coupling strip and offset from the lower coupling strip by an offset amount. In a further embodiment, this method is used to dispose at least two pairs of coupling strips in parallel in different offset amounts. In one embodiment, at least one pair of coupling strips has an offset amount of zero. In another embodiment, at least two pairs of coupling strips have predetermined offset amounts.
In yet another aspect, the invention provides a method for detecting misalignment of layers in a multilayer device by providing a device having at least two first reference probes disposed parallel to each other and spaced by a predetermined amount and a second layer including at least two second reference probes disposed parallel to each other, spaced by a different predetermined amount, and disposed above the first reference probes, Misalignment may then be detected by measuring an amount of coupling between at least two first reference probes and corresponding second reference probes, comparing the amounts of coupling measured for different reference probes, and determining the misalignment of the first layer with the second layer.
In a related embodiment, the invention provides a method for detecting misalignment of layers in a multilayer device, comprising providing a device including a coupler array as described above, measuring amounts of coupling in at least two pairs of coupling strips, comparing the amounts of coupling measured in different pairs of coupling strips, and determining the relative alignment of the first layer with the second layer.
In yet another aspect, the invention provides a system for measuring misalignment of layers in a multilayer device. The system includes a coupler array of claim as described above, a microwave generator, and a microwave receiver to measure the amount of coupling in a pair of coupling strips.
In another aspect, the invention provides a method for measuring misalignment of layers in a multilayer device, by providing a coupler array as described above, providing a microwave generator and receiver, connecting the generator and receiver to a first pair of coupling strips, measuring the amount of coupling in the first pair of coupling strips, coupling the generator to a second pair of coupling strips, measuring the amount of coupling in the second pair of coupling strips, comparing the amount of coupling in the first pair of coupling strips to the amount of coupling in the second pair of coupling strips, and determining the misalignment of layers in the multilayer device.